What is the python script to export the selected meshes in .obj?
export obj to relative file path script:

import bpy
import os

blend_file_path = bpy.data.filepath
directory = os.path.dirname(blend_file_path)
target_file = os.path.join(directory, 'myfile.obj')

bpy.ops.export_scene.obj(filepath=target_file)


further export obj options:

bpy.ops.export_scene.obj(filepath="", check_existing=True, axis_forward='-Z', axis_up='Y', filter_glob="*.obj;*.mtl", use_selection=False, use_animation=False, use_mesh_modifiers=True, use_edges=True, use_smooth_groups=False, use_smooth_groups_bitflags=False, use_normals=True, use_uvs=True, use_materials=True, use_triangles=False, use_nurbs=False, use_vertex_groups=False, use_blen_objects=True, group_by_object=False, group_by_material=False, keep_vertex_order=False, global_scale=1, path_mode='AUTO')